The present invention relates to a device for controlling the switchover of a regular and a standby control board automatically and stably without malfunctions.
It is a common practice with the above-described type of switching control device to removably connect a regular and a standby control board to each other and provide each of them with an MPU (Micro Processing Unit) function. Automatic switching circuits are included in the switching control device for automatically substituting the standby control board for the regular control board, as needed. The switching circuits are implemented by two flip-flops each being included in a respective one of the two control boards and constituted by a buffer and a NAND gate, and LEDs (Light Emitting Diodes) or similar indicators for displaying the status of each of the associated control boards. Assume that an error has occurred in the regular control board in operation. Then, an alarm signal being applied to the NAND gate of the regular control board as a switch command signal goes low to activate the switching circuit. As a result, the regular control board is killed with the LED thereof turned off. At the same time, an alarm signal being fed to the NAND gate of the standby control board goes high to start up the standby control board while turning on the associated LED. In this manner, the two control boards are automatically switched over by the switch command signals, i.e., switch control signals fed from the outside.
However, the problem with the conventional switching control device is that when the standby control board, for example, is pulled out from or reconnected to the regular control board for an inspection or maintenance purpose while the latter is in operation, noise is introduced in the switching circuit of the regular control board. Then, the switching circuit of the regular control board and, therefore, the control device itself often malfunctions. This makes it difficult to inspect or maintain the control device.